Data collected in the NCPP are being analysed to determine primary and contributing roles of biological and environmental factors in mental retardation in a population of 37,000 children followed from the prenatal period to age 7. The incidence of severe retardation did not differ by ethnic group, but mild retardation was more frequent among blacks than among whites. Retardation was negatively related to social class. Major neurological problems were more frequent among severely retarded than mildly retarded children, and were more frequent among whites than blacks in both retarded groups. Within ethnic group, the proportion of neurologically involved retarded children increased with social class. Risk factors for mental retardation include urinary tract infections during pregnancy, teen-age pregnancy, clinical signs of perinatal anoxia, and poor psychomotor performance in infancy.